Rheostat



C. M. TUTTLE Oct. 9, 1934.

RHEosTAT Filed Aug. l5, 1931 "IUIIIIIIIIIINIHINIUIIIIIUIIUIIIII lilllllllllllli A30 Z4 IIgrlr/Z@ 53 |H|||||lllllllllllllllllllllnnmnnmuu @Illlllllllmm Patented Oct. 9, 1934 NT OFFICE anEosrAr Clifton M. Tuttle, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a

corporation of New York Application August 15, 1931, Serial No. 557,260 z claims. (ci. aol- 48) My invention relates broadly to apparatus for varying the resistance of an electrical circuit, and more particularly to a method and means whereby a small modulation of energy in one circuit may be 'ltilized'to vary a relatively large` wattage in av second circuit.

One object of my invention is to provide an amplifying device whereby a small amount of energy may accurately control a relatively large amount of power.

Another object of my invention is to provide an improved methodand means for utilizing the moving elements of an electrical meter as a resistance controlling element.

Other objects'of my invention will be understood from the following description when read in connection with the accompanying drawing in which like reference characters denote like parts throughout.

Fig. 1 is a front elevation ofone form in which the resistance construction of my invention may be embodied.

Fig. 2 is a plan view partly diagrammatic of the entire apparatus.

Fig. 3 is a side elevation of the construction shown in' Flg. 2.

Fig. 41s a diagrammatic view showing a modiiied form of construction tion may be embodied.

,3o Fig. 5 ditically illustrates one application of my invention wherein the resistance is employed to regulate the intensity of a light source in accordance with the transparency of a photographic hlm.

The principal part of my invention vmay be termed a variable resistance or rheostat and con- 4sists of a pair of insulated bars 10 and 11 upon which resistance elements comprising turns of wire 12 and 13 are wound. 'Ihese bars are arranged side by side in parallelism and are normally spaced from each other as shown. A contact element 14 movable betweeny and lengthwise j of the bars 10 and 11 is adapted tocontrol the number of turns of wire 12 and 13 connected be- 45 tween the terminal posts 15 and 16 in the folof resistance between these posts.

in which my inven- In Figs. 2 and 3 I have diagrammatically shown one arrangement for moving the contact 14 and for causing the bars 10 and 11 to grip this contact. The movable coil 17 of any suitable electrical instrument 17', here shown in outline, carries a vane 18 at the end of which is mounted the contact element 14 which may be of any suitable conducting material such as lead or silver. The bars 10 and l1 are curved to register with the path of the contact 14 and are mounted on one end of rods 19 by any suitable means such as headed studs 20. The rods 19 at their other end may be rotatably mounted on a small rod 21 and held in spaced relation thereon by a sleeve 22 and nuts 23. This rod 21 is mounted instationary, relation to the electrical instrument 17 in some suitable manner, and since there are numerous obvious ways of doing this, it is not now illustrated. Some suitable means such as springs 24 are employed to bias the bars 10 and 11 in contact gripping p0sition.

Intermediate of their ends, the bars 19 are provided with cam surfaces 25 adapted to cooperate with cams 26 for separating the bars 19 against the tension of the springs 24. 'Ihe cams.

26 are rigidly mounted on the shaft 27 and may be rotated by turning a knob 28. The faces of the cams 26 farthest from the center are slightly flattened so as to be in stable position when turned to hold the bars-10 and 11 in spaced relation.

Instead of having both bars wound with wire as above described, my invention may take the, form shown in Fig. 4. In this form the bar 30 is wound with resistance wire 32 and the bar 31 'is a plain conductor. Also, the bar 30 varies in cross-section from end to end so that unit movement of the contact 14 from one end of the bars 30 and 31 acts to progressively insert less and less resistance. In other words, the resistance is varied as a function of the straight line movement of the contact 14.

From the above description it will be seen that my invention provides a rheostat having a'slider or variable contact which is substantially frictionless and may be actuated by a delicate in' strument sensitive to feeble currents. My improved rheostat may be employed in any electrical circuit where it is desired to vary lthe resistance of the circuit in accordance with variations outside o! the circuit containing the rheostat.

, For example, as illustrated in Fig. 5, the rheostat may be used for regulating the light intensity in photographic printing apparatus. In this ligure the printing apparatus is illustrated employing a fv movable coil 17 of the meter 17' to detee its density and therefore the printing light intensity necessary to properly expose a sensitive emulsion through it in e.v mown interval oi time. The intensity of the light reac' the thermopile 45 and its consequent activation is a function of the density of the negative and will cause the coil 17 to rotate to bring the contact 14 into position relative to the resistance member to insert the proper amount of resistance in the circuit of the lamp 40 to regulateits intensity to the value desired for printing a negative of such density. In order to avoid burning the/contact element 1d I prefer to clamp the bars 10 and il on to it prior to closing the switch 42 and to open the switch t2 prior to moving the bars away from the contact 14. The switch i2 y be closed and opened by hand to time the exposure, or any well known device may be employed to auto= matically control the interval during which the switch is closed.

My invention as described in connection with a photographic printer operates as follows: The negative 43 to be printed is placed between the lamp 44 and the thermopile 45 which causes the contact 14 to move to a position corresponding to the density of the negative, i. e., the activation of the thermopile. The mob 28 is now turned to rotate the cam 26 to the position shown in Fig. 3 permitting the springs 2 to bring the wire turns 12 and -13 intopressed engagement with the contact 14. The negative is thenplaced in printing position between a sensitive layer e6 and the printing lamp 40. Switch 4.2 is now Leraars' p J closed to energize the lamp and isv opened after a predete i interval oi time which is preferably the same for all negatives, the exposure being governed by the intensity ofthe lp all.

It is to be understood that the application oi my invention toa photographic printer is merely illustrative and is not intended to restrict it to said use alone since it is obvious that the meter may be actuated by any'suitable current or potential and the rheostat may form a part of any electrical circuit in which it is desiredrto vary its resistance in accordance with the reading oi suitable indicating instrument.

While I have described my invention and certain limited embodiment it is to be understood that my invention is not limitedlto the particular construction shown, but that various changes may be made within the scope of the appended claims without departing from the-spirit of my invention.

What 1 claim as new and desire to secure by lLetters Patent of the United States is:-

1. In combination, an electrical instrument having a part movable as a result of electrical variation, a conducting element carried by said im movable part, an electrical circuit including two members electricallyl separated from each other and from said element and at least one member being of resistance material, means for urging said members into electrical contact with said m5 element, and means for moving said members away from each other.

2. A variable resistance comprising in combination a pair of spaced insulated bars, resistance wire carried by each bar, a variable contact ar- M@ ranged to move between said spaced bars and ns for moving said bars toward each other to bring said wire into engagement with said contact. 

